A complex medium is a medium consisting of a combination of at least a solid phase and a liquid phase, and generally including a gaseous phase, these phases being interlinked so that the volume elements containing them are separated by interfaces between phases of great extension. These interfaces of great extension modify the thermodynamic properties of the phases and the constituents forming these phases.
Depending on the nature of the solid phase, it is possible to differentiate porous media, which comprise a rigid solid structure, pastes and gels, which exhibit a deformable solid structure, powders, which exhibit a solid structure formed by an assembly of grains or even agri-foodstuff products whose solid phase retains the form of the cellular structure of the biological tissues from which they are derived.
The liquid phase is generally water, but other liquids can also be found.
When the gaseous phase is absent from the complex medium, the medium is then said to be saturated. The complex medium then comprises a solid phase and a liquid phase.
The activity of a liquid is a parameter representative of the physical-chemical bond of this liquid with the complex medium concerned, in other words, of the interaction between this liquid and this complex medium. More specifically, it is defined by the ratio between the partial vapor pressure of the liquid in the complex medium concerned and the saturating vapor pressure of the pure liquid at the same temperature.
The present invention relates in particular to the field of measuring the activity of water in a complex medium.
A device suitable for measuring the activity of water in a complex medium is applicable in the economic business sectors in which water plays an important role, for example in the agri-food industry.
In practice, such a device makes it possible to ensure the control of the quality of the products in the preparation, packaging, storage and transportation phases of food products by checking that the activity of the water does not exceed a critical value beyond which there is a risk of degradation of the products by the growth of microorganisms. Moreover, the knowledge of the activity of the water may make it possible to regulate the taste qualities or the texture of the food products.
It can also be applied in the field of powders, in particular in the pharmaceutical industry. In this case, the measurement of the activity of a liquid is representative of the bond between this liquid and the powder. This datum is of interest in controlling the agglomeration of the powder and, consequently, the production of tablets.
The field of biology and/or biotechnology may be involved. In biofiltration for example, microorganisms are used to produce molecules or to depollute the air or polluted liquids. These micro-organisms have to be kept in optimum conditions by regulating the temperature and the activity of the water for a good hydric feed.
In civil engineering, most construction materials are porous (concretes, plasters, wood). The activity of the water in these materials controls the mass transfer, which conditions the degradation of these materials over time.
In agronomy, the hydric feed to the plants is determined by the activity of the water. Below a certain activity, which depends on the plant, this feed is no longer possible. Furthermore, the chemical reactions in the soil solution which participate in the mineral feed to the plant, are closely dependent on the activity of the water. This activity is therefore an important parameter for good management of the soils.
Many other fields of application can be cited as nonlimiting examples, such as medical engineering, archeology or artwork conservation.
Devices and/or methods are known for measuring the activity of a liquid in such media.
In particular, one method that is known and widely used for measuring the activity of the water is the saturated saline solutions (SSS) method.
This method consists, first of all, in determining the desorption isotherm of a porous medium being studied. The desorption isotherm is the curve representing the trend of the water content (y axis) as a function of the trend of the activity of the water (x axis).
To this end, a sample forming a porous medium is placed in an isotherm enclosure, the sample being suspended above the saturated saline solution of the chosen salt (NaCl, KCl, KOH, etc.). The enclosure is a sealed enclosure making it possible to define an internal volume of this enclosure. In this volume, the activity is regulated precisely by the salt. The temperature is regulated by an external device.
At thermodynamic balance, that is to say, when the mass of the sample stops changing, the activity of the water in the sample is then equal to the activity of the water in the saline solution, and in the gaseous phase. By virtue of the ideal gas law applicable to water vapor, this activity is equal to the relative humidity of the humid air in the enclosure. The measurement of the partial water vapor pressure then leads to the knowledge of the activity of the water in the sample.
The water content in the sample is then measured, in order to obtain a (water content, water activity) pair.
By repeating the preceding steps with various saline solutions, it then becomes possible to establish the complete curve of the desorption isotherm. Table 1 below gives the relative humidity values at balance for different salts used—French standard NF X 15-119.
TABLE 1Relative humidity or activity of theSaltwater, at balance (%)C2H3KO222.5CR2Na2O7, 2H2O51.7K2SO496KOH7.2MgCl33.4NaCl75.3
Once the desorption isotherm has been determined, the activity of the water in this sample can be deduced simply by measuring the water content in the sample.
One drawback with this method is that the time needed to obtain the thermodynamic balance between the saturated saline solution and the sample is lengthy, from a few days to a month when the activity of the water approaches the unity value.
Another drawback with this method is that it is indirect, in as much as it entails first determining the desorption isotherm to obtain the activity of the water in the porous medium.
Now, the determination of a single desorption isotherm does not generally represent, sufficiently accurately, the behavior of a natural medium, the latter being able to exhibit a wide variability. This variability is for example due, in the agri-food domain, to the genetic or varietal origins of the products, or to the methods used to process these products.
Another drawback with this method is that it is limited to measuring the activity of the water. This method cannot therefore relate to applications for which the measurement of the activity of a liquid other than water would be necessary. Such is, for example, the case in the environmental domain, for the depollution of soils where the measurement of the activity of a liquid other than water could make it possible to assess the stabilization of a non-aqueous pollutant (volatile organic compounds VOC, hydrocarbons, etc.).
Moreover, certain devices vary the relative humidity of the air in the enclosure, in which the sample whose activity is to be known is placed, and the sample is weighed each time. To vary the relative humidity, one known method consists, for example, in feeding the enclosure with different water vapors.
There is therefore no need to insert a new saline solution each time a measurement point is performed. Furthermore, since the thermodynamic balance is obtained more rapidly with these methods, the time needed to acquire the isotherm is shorter.
However, although these methods improve the speed of acquisition of the desorption isotherm, they are still, however, indirect and limited to measuring the activity of the water.
Moreover, they require the relative humidity to be regulated in the isotherm enclosure which can prove difficult, and generally less accurate than with the saturated saline solutions method.
To perform a regulation with an accuracy comparable to the saturated saline solutions method, consideration may be given to using devices comprising so-called “dew point” sensors. These sensors are, however, difficult to set up.
Another method for measuring the activity of the water consists in effecting temperature levels in a sealed enclosure containing the sample. The total pressure of the gaseous phase and the relative humidity of the air are measured for each level, once the thermodynamic balance is reached.
Here again, a difficulty with this type of method stems from the measurement of the relative humidity, and from the limitations of the sensor used for this purpose.
Furthermore, the method for measuring the water content that is employed (Kelvin's law) restricts the scope of the method. While this method is applicable for porous media in which the liquid is susceptible to displacement by capillarity, it cannot be used for finer porous media, such as argillaceous soils, or for media for which the liquid/gas interface does not exist, such as agri-food products or biological tissues.